Neutral Models of Coexistence

Recently, there has been a surge of interest in ecology for neutral models of coexistence. In neutral models of coexistence, niche differentiation among species is removed from consideration and species are considered to be equivalent. However, fundamental properties of population dynamics (e.g., probabilities of birth, death, and immigration) and interspecific competition play an important role in determining coexistence. Many of the patterns of coexistence and diversity, species-area relationships, and the distribution of abundances among species in the real world can be explained through neutral models that do not include niche differentiation among species. Frequency-dependent natural selection can also generate coexistence between species that are apparently neutral (i.e., there is no niche differentiation between the species). In this situation, when species are rare (in low-abundance), they adapt to a high frequency of interspecific competitive interactions, thus providing for their persistence. When species are common (in high-abundance), they adapt to a high frequency of intraspe-cific competitive interactions, as they are more likely to encounter conspecifics in the habitat in which they are abundant. Under these conditions, the species are said to have evolved 'pseudo-neutrality'. Nevertheless, the

Table 1 Some models and explanations of processes that promote coexistence among species and individuals within habitats

Model

Explanation

Example

Lottery

Recruitment limitation Niche diversification Tradeoffs

Competitive networks Predation limitation

Abiotic disturbance

Intraspecific aggregation Asynchronous life histories

Neutral theory

Available space occupied at random from larval/ propagule species pool

As above, but habitat occupation limited by patchy supply of larvae/propagules Many available microhabitats promote coexistence

High performance in one life-history feature at the expense of reduced performance in another

No species is dominant over all others

High predation (esp. on common prey-switching)

promotes coexistence Physical factor(s) that create space for colonization/

replenishment Species undergo higher intraspecific than interspecific competition

Seasonal resource use asynchronous among species, promoting coexistence (e.g., reproductive cycling, population irruptions) No niche diversification among species, probabilities of birth, death, and immigration and interspecific competition determine coexistence

Species with dispersive larvae/propagules (e.g., coral reef fishes) occupying space at random

Marine benthic organisms esp., at edge of their range

Different plant parts used by invertebrates as a resource

Plants either produce many small seeds (good colonizing ability) or few large seeds (good competitive ability)

Overgrowing corals

Predation on lower rocky intertidal marine environments

Storm gaps in rainforest canopies

Patchily distributed and aggregated plant populations of different species

Cicadas

High diversity of plant species in rainforests problem is that a community theory that accounts neither for species differences nor for patterns of species distribution across real physical geography must be incomplete.

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